Some examples of hydrozoans are:
– Hydra
– Obelia
– Portuguese Man O’ War
It enables the Portuguese Man O’ War to stay at the current water depth, ascend, or descend without having to waste energy in swimming.
The air bladder is known as the ‘pneumatophore’ or ‘sail’. The Portuguese Man o’ War has no means of propulsion and is pushed by the winds and the current. Their bladder must stay wet to ensure survival. Every so often it may roll slightly to wet the surface of the float. To escape a surface attack, the pneumatophore can be deflated allowing the Man O’ War to briefly submerge.
Below the main body dangle long tentacles, sometimes reaching 10 metres (33 feet) in length below the surface, although 1 metre (3 feet) is the average. They sting and kill small sea creatures such as small fish and shrimp using venom-filled nematocysts.
The sting from the tentacles is dangerous to humans. These stings have been responsible for several deaths, however, they usually only cause excruciating pain. Detached tentacles and specimens which wash up on shore can sting just as painfully as the intact creature in the water for weeks after their detachment. The venom can travel up to the lymph nodes and may cause, depending on the amount of venom, more intense pain. In extreme cases medical attention is necessary.
